Message-ID: <CAMgjq7C_nVynRpMV8xkyVuhpyDY6qZX_ShzxChen5Fh5gXSJVg@mail.gmail.com>
Date: Tue, 3 Jun 2025 19:48:49 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Andrea Arcangeli <aarcange@...hat.com>, David Hildenbrand <david@...hat.com>, 
	Lokesh Gidra <lokeshgidra@...gle.com>, stable@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: userfaultfd: fix race of userfaultfd_move and swap cache

On Tue, Jun 3, 2025 at 6:08 AM Barry Song <21cnbao@...il.com> wrote:
>
> On Tue, Jun 3, 2025 at 8:34 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Tue, Jun 03, 2025 at 02:14:19AM +0800, Kairui Song wrote:
> > > From: Kairui Song <kasong@...cent.com>
> > >
> > > On seeing a swap entry PTE, userfaultfd_move does a lockless swap cache
> > > lookup, and try to move the found folio to the faulting vma when.
> > > Currently, it relies on the PTE value check to ensure the moved folio
> > > still belongs to the src swap entry, which turns out is not reliable.
> > >
> > > While working and reviewing the swap table series with Barry, following
> > > existing race is observed and reproduced [1]:
> > >
> > > ( move_pages_pte is moving src_pte to dst_pte, where src_pte is a
> > >  swap entry PTE holding swap entry S1, and S1 isn't in the swap cache.)
> > >
> > > CPU1                               CPU2
> > > userfaultfd_move
> > >   move_pages_pte()
> > >     entry = pte_to_swp_entry(orig_src_pte);
> > >     // Here it got entry = S1
> > >     ... < Somehow interrupted> ...
> > >                                    <swapin src_pte, alloc and use folio A>
> > >                                    // folio A is just a new allocated folio
> > >                                    // and get installed into src_pte
> > >                                    <frees swap entry S1>
> > >                                    // src_pte now points to folio A, S1
> > >                                    // has swap count == 0, it can be freed
> > >                                    // by folio_swap_swap or swap
> > >                                    // allocator's reclaim.
> > >                                    <try to swap out another folio B>
> > >                                    // folio B is a folio in another VMA.
> > >                                    <put folio B to swap cache using S1 >
> > >                                    // S1 is freed, folio B could use it
> > >                                    // for swap out with no problem.
> > >                                    ...
> > >     folio = filemap_get_folio(S1)
> > >     // Got folio B here !!!
> > >     ... < Somehow interrupted again> ...
> > >                                    <swapin folio B and free S1>
> > >                                    // Now S1 is free to be used again.
> > >                                    <swapout src_pte & folio A using S1>
> > >                                    // Now src_pte is a swap entry pte
> > >                                    // holding S1 again.
> > >     folio_trylock(folio)
> > >     move_swap_pte
> > >       double_pt_lock
> > >       is_pte_pages_stable
> > >       // Check passed because src_pte == S1
> > >       folio_move_anon_rmap(...)
> > >       // Moved invalid folio B here !!!
> > >
> > > The race window is very short and requires multiple collisions of
> > > multiple rare events, so it's very unlikely to happen, but with a
> > > deliberately constructed reproducer and increased time window, it can be
> > > reproduced [1].
> > >
> > > It's also possible that folio (A) is swapped in, and swapped out again
> > > after the filemap_get_folio lookup, in such case folio (A) may stay in
> > > swap cache so it needs to be moved too. In this case we should also try
> > > again so kernel won't miss a folio move.
> > >
> > > Fix this by checking if the folio is the valid swap cache folio after
> > > acquiring the folio lock, and checking the swap cache again after
> > > acquiring the src_pte lock.
> > >
> > > SWP_SYNCRHONIZE_IO path does make the problem more complex, but so far
> > > we don't need to worry about that since folios only might get exposed to
> > > swap cache in the swap out path, and it's covered in this patch too by
> > > checking the swap cache again after acquiring src_pte lock.
> >
> > [1]
> >
> > >
> > > Testing with a simple C program to allocate and move several GB of memory
> > > did not show any observable performance change.
> > >
> > > Cc: <stable@...r.kernel.org>
> > > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > > Closes: https://lore.kernel.org/linux-mm/CAMgjq7B1K=6OOrK2OUZ0-tqCzi+EJt+2_K97TPGoSt=9+JwP7Q@mail.gmail.com/ [1]
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > >
> > > ---
> > >
> > > V1: https://lore.kernel.org/linux-mm/20250530201710.81365-1-ryncsn@gmail.com/
> > > Changes:
> > > - Check swap_map instead of doing a filemap lookup after acquiring the
> > >   PTE lock to minimize critical section overhead [ Barry Song, Lokesh Gidra ]
> > >
> > > V2: https://lore.kernel.org/linux-mm/20250601200108.23186-1-ryncsn@gmail.com/
> > > Changes:
> > > - Move the folio and swap check inside move_swap_pte to avoid skipping
> > >   the check and potential overhead [ Lokesh Gidra ]
> > > - Add a READ_ONCE for the swap_map read to ensure it reads a up to dated
> > >   value.
> > >
> > >  mm/userfaultfd.c | 23 +++++++++++++++++++++--
> > >  1 file changed, 21 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > index bc473ad21202..5dc05346e360 100644
> > > --- a/mm/userfaultfd.c
> > > +++ b/mm/userfaultfd.c
> > > @@ -1084,8 +1084,18 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> > >                        pte_t orig_dst_pte, pte_t orig_src_pte,
> > >                        pmd_t *dst_pmd, pmd_t dst_pmdval,
> > >                        spinlock_t *dst_ptl, spinlock_t *src_ptl,
> > > -                      struct folio *src_folio)
> > > +                      struct folio *src_folio,
> > > +                      struct swap_info_struct *si, swp_entry_t entry)
> > >  {
> > > +     /*
> > > +      * Check if the folio still belongs to the target swap entry after
> > > +      * acquiring the lock. Folio can be freed in the swap cache while
> > > +      * not locked.
> > > +      */
> > > +     if (src_folio && unlikely(!folio_test_swapcache(src_folio) ||
> > > +                               entry.val != src_folio->swap.val))
> > > +             return -EAGAIN;
> > > +
> > >       double_pt_lock(dst_ptl, src_ptl);
> > >
> > >       if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> > > @@ -1102,6 +1112,15 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> > >       if (src_folio) {
> > >               folio_move_anon_rmap(src_folio, dst_vma);
> > >               src_folio->index = linear_page_index(dst_vma, dst_addr);
> > > +     } else {
> > > +             /*
> > > +              * Check if the swap entry is cached after acquiring the src_pte
> > > +              * lock. Or we might miss a new loaded swap cache folio.
> > > +              */
> > > +             if (READ_ONCE(si->swap_map[swp_offset(entry)]) & SWAP_HAS_CACHE) {
> >
> > Do we need data_race() for this, if this is an intentionally lockless read?
>
> Not entirely sure. But I recommend this pattern, borrowed from
> zap_nonpresent_ptes() -> free_swap_and_cache_nr(),
> where the PTL is also held and READ_ONCE is used.
>
>                 if (READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
>                        ..
>                         nr = __try_to_reclaim_swap(si, offset,
>                                                    TTRS_UNMAPPED | TTRS_FULL);
>
>                         if (nr == 0)
>                                 nr = 1;
>                         else if (nr < 0)
>                                 nr = -nr;
>                         nr = ALIGN(offset + 1, nr) - offset;
>                 }

Thanks for the explanation, I also agree that holding PTL here is good
enough here.

>
> I think we could use this to further optimize the existing
> filemap_get_folio(), since in the vast majority of cases we don't
> have a swapcache, yet we still always call filemap_get_folio().
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index bc473ad21202..c527ec73c3b4 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
>
> @@ -1388,7 +1388,7 @@ static int move_pages_pte(struct mm_struct *mm,
> pmd_t *dst_pmd, pmd_t *src_pmd,
>                  * folios in the swapcache. This issue needs to be resolved
>                  * separately to allow proper handling.
>                  */
>
> -               if (!src_folio)
> +               if (!src_folio & (swap_map[offset] & SWAP_HAS_CACHE))
>                         folio = filemap_get_folio(swap_address_space(entry),
>                                         swap_cache_index(entry));
>                 if (!IS_ERR_OR_NULL(folio)) {
>
> To be future-proof, we may want to keep the READ_ONCE to ensure
> the compiler doesn't skip the second read inside move_swap_pte().

Maybe we can do this optimization in another patch I think.

>
> >
> > Another pure swap question: the comment seems to imply this whole thing is
> > protected by src_pte lock, but is it?
> >
> > I'm not familiar enough with swap code, but it looks to me the folio can be
> > added into swap cache and set swap_map[] with SWAP_HAS_CACHE as long as the
> > folio is locked.  It doesn't seem to be directly protected by pgtable lock.
> >
> > Perhaps you meant this: since src_pte lock is held, then it'll serialize
> > with another thread B concurrently swap-in the swap entry, but only _later_
> > when thread B's do_swap_page() will check again on pte_same(), then it'll
> > see the src pte gone (after thread A uffdio_move happened releasing src_pte
> > lock), hence thread B will release the newly allocated swap cache folio?
> >
> > There's another trivial detail that IIUC pte_same() must fail because
> > before/after the uffdio_move the swap entry will be occupied so no way to
> > have it reused, hence src_pte, even if re-populated again after uffdio_move
> > succeeded, cannot become the orig_pte (points to the swap entry in
> > question) that thread B read, hence pte_same() must check fail.
>
> in v1 of this patch, we had some similar discussions [1][2]:
>
> [1] https://lore.kernel.org/linux-mm/CAGsJ_4wBMxQSeoTwpKoWwEGRAr=iohbYf64aYyJ55t0Z11FkwA@mail.gmail.com/
> [2] https://lore.kernel.org/linux-mm/CAGsJ_4wM8Tph0Mbc-1Y9xNjgMPL7gqEjp=ArBuv3cJijHVXe6w@mail.gmail.com/
>
> At the very least, [2] is possible, although the probability is extremely low.
>
> "It seems also possible for the sync zRAM device.
>
>  step 1: src pte points to a swap entry S without swapcache
>  step 2: we call move_swap_pte()
>  step 3: someone swap-in src_pte by sync path, no swapcache; swap slot
> S is freed.
>              -- for zRAM;
>  step 4: someone swap-out src_pte, get the exactly same swap slot S as step 1,
>              adds folio to swapcache due to swapout;
>  step 5: move_swap_pte() gets ptl and finds page tables are stable
> since swap-out
>              happens to have the same swap slot as step1;
>  step 6: we clear src_pte, move src_pte to dst_pte; but miss to move the folio.
>
> Yep, we really need to re-check pte for swapcache after holding PTL.
> "
>
> Personally, I agree that improving the changelog or the comments
> would be more helpful. In fact, there are two bugs here, and Kairui’s
> changelog clearly describes the first one.

Yeah, the first one is quite a long and complex race already, so I
made the description on the second issue shorter. I thought it
wouldn't be too difficult to understand given the first example. I can
add some more details.

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